The invention relates to a refrigerator for temperatures below the .lambda.-point of helium. A reservoir for liquid .sup.4 He I is connected, via a connection duct which successively includes, viewed from the reservoir, one side of a heat exchanger in which liquid .sup.4 He I is subjected to a temperature decrease, and a choke member in which the .sup.4 He is subjected to a pressure decrease, to an evaporation chamber for liquid .sup.4 He II. The evaporation chamber is provided with a discharge duct for .sup.4 He gas in which the other side of the heat exchanger is included. The invention furthermore relates to a method of generating temperatures below the .lambda.-point of helium.
In a refrigerator of the kind which is the subject of the present invention and which is known from U.S. Pat. No. 3,427,817, the reservoir normally contains a .sup.4 He-bath of 4.2.degree.K under atmospheric pressure. This liquid .sup.4 He in the normal phase (.sup.4 He I) is subjected to a temperature decrease in the heat exchanger and to a pressure decrease in the choke member, part of the liquid then evaporating. The remaining liquid reaches the evaporation chamber where heat is taken up from the object to be cooled. The temperature to which the object is cooled depends on the pressure in the evaporation chamber. The lower the pressure is, the lower the temperature will be. Via the discharge duct, the evaporation chamber is pumped down to a low pressure by a pumping device.
An advantage of the known refrigerator is that it can operate without interruption for a prolonged period of time, in that it is possible to supply .sup.4 He continuously from the reservoir to the evaporation chamber. However, if cooling temperatures below the .lambda.-point of helium (2.18.degree.K) are to be realized (for example, for condensing concentrated .sup.3 He in .sup.3 He-.sup.4 He dilution refrigerators or for cooling masers, computer stores, susperconductive coils, paramagnetic salts etc,), problems arise, so that the known refrigerator cannot be used at all or only with great difficulty.
The cause of these problems lies in the fact that when liquid He II, having its superfluid properties, is present in the evaporation chamber (.sup.4 He below 2.18.degree.K), this He II creeps upwards as a thin film on the wall of the chamber, through the choke member to the region of higher temperature. This flowback of helium occurs in spite of the opposing force of gravity and in spite of the opposing force caused by the higher pressure level on the inlet side of the choke member. As a result, He II cannot be permanently stored in the evaporation chamber, so that the operation of the refrigerator is disturbed.